A lithographic process is one that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Stepping and/or scanning movements can be involved, to repeat the pattern at successive target portions across the substrate. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is necessary to make very accurate position measurements over a substrate, to ensure proper alignment between patterns formed in different patterning steps, for example between two layers in a device. Accurate position sensors have been developed which use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. One type of position sensor is based on a self-referencing interferometer, described for example in US2004033426A1. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in US2015261097A1. Modified alignment marks have also been developed, for example as disclosed in US2009195768A1, The contents of these publications are incorporated herein by reference. The present disclosure is not limited to this specific type of interferometer.
In many applications, optical properties of the substrate and alignment marks make it difficult to obtain a high-quality signal. For example, as the manufacturing process progresses, it may be necessary to read an alignment mark which is buried under various layers of material. Photocurrents obtained by photodetectors within the optical system of the position sensing arrangement become very weak, and difficult to distinguish from electronic noise. The sensors which are employed in state-of-the-art lithographic apparatuses use several techniques to obtain the best quality signal, for example by selecting from a range of different wavelengths and polarizations of radiation, to find the one which best penetrates layers overlying the alignment mark. Nevertheless, requirements for accuracy are ever-increasing, while the structures to be measured become more complex. One way to improve signal quality is by taking more time for the measurement. However, this may be undesirable because of the requirement to maintain a very high throughput in a commercial lithographic apparatus.
Other types of position sensors are known, and are also subject of the present disclosure. One example is the ATHENA sensor as disclosed in U.S. Pat. No. 6,297,876B1 (Bornebroek et al) and the laser interferometric alignment (LIA) sensor disclosed in U.S. Pat. No. 6,285,455B1 (Shiraishi).
In an unrelated field, that of quantum optics, a technique of balanced heterodyne detection has been developed to allow very sensitive detection of single photon events with high detection bandwidths. These techniques are described for example in the publications U Leonhardt, Measuring the quantum state of light, Cambridge University Press (1997); S RHuisman, et al, Opt. Lett., 34, p 2739 (2009); and A I Lvovsky and M G Raymer, Rev. Mod. Phys., 81, p 299 (2009). The contents of these publications are incorporated herein by reference. Note that “bandwidth” in this context relates to the bandwidth of signals being processed, not the bandwidth of the radiation used to obtain those signals.